Pioneer Anomaly

Latest news (June 2012): The Pioneer Anomaly appears
to have been solved in a paper published in the journal Physical
Review Letters. Jump ahead to find the
solution.

Introduction to the puzzle

The Pioneer 10 and Pioneer
11 deep space probes were not where they were supposed to be. Radio
telemetry received from the spacecraft, over a period of many years before
contact was lost with both probes, indicated that they were slowing down
slightly more than expected with the result that each year they traveled
about 5,000 km (3,000 miles) less than mission controllers had projected.
The slowing down was tiny, amounting to a deceleration toward the Sun of
(8.74 ± 1.33) 10-10 m/s2, i.e. than a nanometer
(a billionth of a meter) per second per second. This is equivalent to just
one ten-billionth of the gravity at Earth's surface. However, although incredibly
small, the effect was persistent over several decades. When mission controllers
last heard from Pioneer 10, it was a quarter of a million miles from where
it was supposed to be – roughly the distance from the Earth to the
Moon. When NASA lost touch with Pioneer 11, several years earlier, it was
heading for a similar deviation.

The anomalous deceleration of the twin Pioneers was inferred from a small,
constant, anomalous Doppler shift in the frequency of radio signals received
from the spacecraft. Various explanations were put forward to account for
the so-called Pioneer Anomaly. Although it was always suspected that there
might be a systematic origin to the effect, such as thrust from a gas leak,
none had been found. Some scientists therefore began to consider more exotic
explanations, including the possibility that our understanding of gravity
might need to be revised.

Origins
of the mystery

The trajectories of Pioneer's 10
and 11 and Voyager's 1 and 2 up to 1992

Pioneer 10 was launched in 1972 and became the first spacecraft to fly past Jupiter. Its identical sister craft, Pioneer
11, was launched the following year and subsequently flew past both Jupiter
and Saturn. Both are now heading out of the
Solar System but in completely different directions. For details of the
spacecraft and where they are heading, see the pages on the respective probes.

From about 1980 on, evidence began to accumulate for what became known as
the Pioneer Anomaly. This evidence was first brought to light and, was subsequently
analyzed, by John Anderson (Pioneer 10 Principal Investigator for Celestial
Mechanics) and his colleagues at the Jet Propulsion Laboratory in Pasadena.
Their first published results appeared in 1998; 1 these prompted
a number of articles in publications such as Scientific American (December 1998 issue) and Newsweek magazine (October 4, 1999 issue)
which brought the phenomenon to widespread public attention. Anderson and
coworkers wrote a second paper, which appeared in 2002, discussing the anomaly
further.2

The JPL team pointed to data from the Ulysses and Galileo spacecraft which suggested that
a similar effect might be at work on these probes. However, it was hard
to draw firm conclusions in the case of Ulysses and Galileo partly because
they hadn't traveled as far from the Sun as had Pioneers 10 and 11. The Voyager 1 and Voyager
2 spacecraft, which had now gone even further from the Sun than the
Pioneers, weren't a helpful as might have been expected in contributing
to the investigation of the Pioneer anomaly because of the way in which
they are stabilized. Unlike the Pioneers, which are spin-stabilized, the
Voyagers have what is known as three-axis
stabilization. This results in an greater uncertainty in the spacecrafts'
theoretical positions. The uncertainty was great enough to mask any deceleration
similar in magnitude to that seen in the Pioneer probes.

Possible explanations

A variety of possible causes of the Pioneer anomaly were considered, ranging
from the mundane to the exotic. These included:

observational errors, including measurement and computational errors,
in deriving the acceleration

Until 2012, it wasn't known which, if any, of these explanations was correct.
No spacecraft behavior or previously unknown property of the outer solar
system had been able to explain the anomalous decelerations. Hence, although
scientists expected that in the end a mundane explanation was most likely,
they were forced to consider theories that involved a change in our understanding
of the laws of physics.

Further research

One problem hampering researchers early on was that the only data available
to them about the movements of the Pioneers was from 1987 onward. The reason
for this is that earlier data received from the probes was stored on magnetic
tapes in a format that could only be handled by antiquated computers. NASA
was unwilling at the time to allocate resources to analyze this data. Consequently,
Anderson and his colleagues approached the Planetary Society, requesting
funds in order that the data could be recovered and made available to researchers
wishing to look further into the Anomaly. The Planetary Society appealed
to its members and received the support needed for the recovery to go ahead.

At the same time, other lines of research were proposed, such as close observation
of other spacecraft heading for the outer solar system, including New
Horizons, which is bound for Pluto and the Kuiper Belt. Unfortunately,
New Horizons suffered from a similar drawback in this respect to the Cassini spacecraft – namely, that its RTGs are mounted close to the spacecraft's
body, so infrared radiation from them, bouncing off the spacecraft, would
produce a systematic thrust of a not-easily predicted magnitude, several
times as large as the Pioneer effect.

Another suggestion, made by Gary Page of George Mason University and his
colleagues, was to use remote asteroids to test if an unknown gravitational
factor is the cause of the anomaly. They identified 15 asteroids that they
considered might be subject to the mysterious force; all of the asteroids'
orbits stretched far into the outer solar system where the anomaly had made
itself felt.

Mystery solved

In 2004, with the money provided by the Planetary Society and some funds
which eventually came in from NASA, a group of researchers, led by Slava
Turyshev, at JPL, started to gather, extract, and analyze all the data held
in earlier records of the Pioneer probes. At the back of their minds was
the possibility of proposing a new deep space mission to investigate the
Anomaly if they couldn't find an answer to the mystery.

Turyshev and his colleagues copied digitized files from the original navigation
computer that had helped steer the Pioneers, found more than a dozen boxes
of magnetic tape under a staircase at JPL, and received archived files from
other NASA centers. They even managed to save an old tape machine from being
scrapped in order to play the tapes.

From these various sources, the researchers were able to extract enough
Doppler and telemetry data, stretching back to the launch of the spacecraft,
to solve the puzzle once and for all. It turned out that the heat given
off by the electrical subsystems of the probes and the decay of their plutonium
thermoelectric power sources was just right to explain the puzzling deceleration.
The spacecrafts' own heat emanations were what was pushing them back, very,
very slightly. As Turyshev explained: "The effect is something like when
you're driving a car and the photons from your headlights are pushing you
backward." The results were published in the June 12, 2012 issue of Physical
Review Letters.[3]